"Bridge failure." Those two words strike fear into engineers’ hearts everywhere. In Minneapolis, they’ve taken on new meaning. When the I-35 West bridge broke down over the Mississippi River, the government started a sweeping investigation into U.S. infrastructure.
Most of our "modern" bridges are in fact 40 to 50 years old. They were built of steel and concrete with little knowledge that steel is subject to miniscule stress cracks in their very crystalline structure, and that concrete won’t hold it all together for long after the steel fails.
Truss bridges, such as the I-35 bridge, are built on arches, which support the deck, or driving surface. Vertical "trusses" – columns that bear most of the stress of the load – often reinforce the arches and the deck. When a truss snaps, that stress has to be shunted to another part of the bridge seamlessly. This almost never happens, unless the engineer that designed the bridge allowed for redundant support. Most don’t, as it adds cost and complexity to the project.
This means that if a single truss fails, the whole bridge fails, often with catastrophic results.
Span bridges, like most freeway overpasses, are also subject to collapse in the same manner. A span bridge places the deck on columns or pillars, and is held up by the structure of the deck, typically steel-reinforced concrete.
There is a span bridge on Highway 288 South over Sims Bayou that is failing slowly but surely. The sand bank that shores up the pilings is clearly eroding, and the concrete embankment is cracked and buckling. There is no major damage to the deck, but the problem is still apparent. The embankment has crumbled away from the guardrail, and could very well drag the guard rail down if it falls any further. There is exposed rebar.
The Army Corps of Engineers is working on a reclamation project nearby, which makes the repair project dicey until it is complete.
TxDOT has assigned a crew to work on it, and the demolition phase has begun.
The most important thing that can be done to fix the problem is a thorough inspection of all bridges in this age range nationwide, followed by a complete repair if needed. The closer the inspection, the better the results, and lives saved always outweigh the costs of the projects. Any government – county, state or otherwise – would be foolish not to heed that.
Conant, an entrepreneurship freshman, can be reached via [email protected]
